Make-and-break spark plug

ABSTRACT

There is disclosed a make-and-break type of spark plug for piston engines comprising a spark plug housing equipped with a counter electrode and a movable electrode positioned to cooperate with said counter electrode to form therebetween a short-circuit path. An armature is movably arranged within the spark plug housing and a magnetic coil acts upon said armature. Means serve to connect the armature with the movable electrode, and said armature includes an end face directed away from said movable electrode with a pressure equalization compartment being located within said spark plug housing at the region of said end face of said armature facing away from said movable electrode. At least one bore communicates the pressure equalization compartment with the interior of an engine cylinder associated with the spark plug, said at least one bore extending through said armature, said connecting means for said armature with said movable electrode and through both of said electrodes.

Elnited States Patent Pasbrig 1 1451 Sept. 26, 1972 {54] MAKE-AND-BREAKSPARK PLUG [72] Inventor: Max Pasbrig, Casa Luce, Via Eco,

6644 Orselina, Switzerland [22] Filed: Jan. 11, 1971 [21] Appl. No.:105,403

52 U.S. c1 ..123/143 R, 123/1465 R, 123/169 R, 123/169 CL, 123/169 EL,313/152 51 Int, Cl ..F02p 23/00 [58] Field of Search ..l23/l43 R, 146.5R, 169 R, 123/169 CL, 169 EL, 169 EA; 13/4; 313/152 PrimaryExaminer-Laurence M. Goodridg Attorney-Werner W. Kleeman [5 7] ABSTRACTThere is disclosed a make-and-break type of spark plug for pistonengines comprising a spark plug housing equipped with a counterelectrode and a movable electrode positioned to cooperate with saidcounter electrode to form therebetween a short-circuit path. An armatureis movably arranged within the spark plug housing and a magnetic coilacts upon said armature. Means serve to connect the armature with themovable electrode, and said armature includes an end face directed awayfrom said movable electrode with a pressure equalization compartmentbeing located within said spark plug housing at the region of said endface of said armature facing away from said movable electrode. At leastone bore communicates the pressure equalization compartment with theinterior of an engine cylinder associated with the spark plug, said atleast one bore extending through said armature, said connecting meansfor said armature with said movable electrode and through both of saidelectrodes.

12 Claims, 3 Drawing Figures MAKE-AND-BREAK SPARK PLUG BACKGROUND OF THEINVENTION The present invention relates to a new and improvedmake-and-break type spark plug for piston engines, which is of the typewhere a magnetic coil acts upon an armature actuating a movableelectrode and at which an ignition pulse passes through the magneticcoil and a short-circuit path betweenthe electrode and a counterelectrode (ground electrode), whereby the magnetic field building up inthe magnetic coil brings about a socalled tearing up or breaking away ofthe short-circuit path under corresponding spark formation.

Make-and-break spark plugs possess the advantage in contrast toconventionally employed spark plugs with a fixed electrode that they canproduce powerful ignition sparks at relatively low voltages. At a timewhere the nowadays employed high ignition voltages are still poorlycontrolled attempts have again been made to employ in practicemake-and-break spark plugs, yet without these efforts having broughtabout any extensive success. The reason therefore is probably based uponthe fact that the problems arising by virtue of the burn-off, thecharring or pitting of the interrupter contacts, the soot build-up andsimilar disturbances could only be partially overcome, and in particulargreat difficulties arose because of the pronounced and periodic alteringpressure. Above all, these difficulties are predicated upon the factthat the movable electrode must be torn away by the armature of themagnetic coil from the fixed or stationary electrode in a rapid timesequence, and specifically corresponding to the rotational speed of thepiston engine, so that the additional difficulty arose that the breakingor tearing away movement must occur against the compression force,requiring the armature and the magnetic coil to exert relatively highforces.

SUMMARY OF THE INVENTION Accordingly, there is still present a real needin the art for an improved make-and-break spark plug which is notassociated with the aforementioned drawbacks of the prior artconstructions. Therefore, a primary objective of the present inventionis to provide such type spark plug which effectively and reliablyfulfills the existing need and is not associated with the previouslyexplained drawbacks.

In consideration of the foregoing explained problem of thestate-of-the-art spark plugs of this type a further specific object ofthe present invention therefore is concerned with reducing the forceswhich must be exerted by the armature for the purpose of actuating themovable electrode and to that end especially making use of the pressurein the internal cylinder compartment, that is to say, the compressionpressure.

Now, in order to implement these and still further objects of theinvention, which will become more readi- By virtue of the pressureequalization afforded by the teachings of the invention the armatureforces required for tearing away of the movable electrode areconsiderably reduced. The requisite armature force must essentially onlyapply the necessary inertia force for shifting away the movableelectrode and essentially need not overcome any additional pressureforces since such are almost completely balanced out by the pressureequalization chamber and the pressure equalization bore means.

The pressure increasing to a relatively high value within the cylindershould, on the one hand, lightly press the electrode against the counterelectrode so that a pronounced magnetic field builds up at the magneticcoil before the armature raises the electrode against the action of thisforce and the stress of an associated spring from the counter electrode.However, on the other hand this pressure action should not reach amagnitude where lifting or raising of the electrode is questionable. Atlow voltages a small gap between both electrodes constitutes arelatively high electrical resistance. Since the magnetic coil and thisgap are located in series this means that the current delivered to thespark plug drops quite markedly already at the beginning of the tearingaway or contact break movement. Therefore, the magnetic field should becompletely built-up already at the beginning of such contact-breakmovement and have reached its optimum value. However, when thecontact-break movement has once been started, then, the compressiveforce built-up in the cylinder, and if necessary, together with theinitiated combustion, should support the contact-break movement, so thatthe movable electrode can assume, as rapidly as possible, its favorablespacing from the fixedelectrode. The collapsing magnetic field and, asthe case may be in conjunction with the still flowing current thendelivers a series of pronounced ignition sparks which distribute overthe ring-shaped electrode surface. The tearing away or contact-breakmovement therefore should occur, as contemplated by the invention, inthe form of a catapulting movement.

The thermal value or thermal coefficient does not play any, or only avery slight, role for the inventive spark plug. It need not be fearedthat the spark plugs of the engine, which normally require a smallthermal coefficient, will be soiled, while, on the other hand, the sparkplug is also suitable for motors with a high thermal coefficient,without having to fear overheating. This is predicated upon the factthat the electrode is ring-shaped and in comparison to conventionalspark plugs or to heretofore known make-and-break spark plugs is quitelarge. The forceful current flow prior to the contact-break movement,the catapulting of the electrode, and the relatively forceful reboundtakes care of possible combustion residues. The thermal conductivitybetween the electrode and the counter electrode affords rapid heatwithdrawal.

The movement of the armature and the electrode occurs axially withrespect to the cylinder opening, the movable electrode upon tearing-awayor breaking the short circuit path being catapulted or displaced in thedirection of the interior of the cylinder away from the counterelectrode.

According to a preferred embodiment of the invention the electrodeextends past the periphery of the electrode support providing theconnection with the armature and bears with its protruding marginalportion, in its rest position at a neighboring end surface of the sparkplug housing forming the counter electrode.

In each instance excitation of the magnetic coil is periodically broughtabout by current pulses controlled by a thyristor as a function of therotational speed of the engine, so that a series of ignition or firingsparks which continuously wander upon the surface of the electrodeappear between the movable electrode and the neighboring end surface ofthe spark plug housing.

The electrode should be formed of a suitable materi- -al possessing avery high molecular weight and a melting point exceeding 3,000 C. It ispreferred to use molybdenum or a molybdenum alloy having a melting pointof about 3,4l C. I 4

According to a preferred embodiment of the invention the contact surfacebetween the movable electrode and the counter electrode, which contactsurface forms the short-circuit path is constructed to possess asubstantially ring-shaped configuration, the external diameter of thering-shaped contact surface possessing, for instance, a value of atleast millimeters. The largest contact surface furthermore can possessan area of at least 20 mm.

The voltage of the ignition pulse preferably amounts to 30 to 50 voltswhereas the current intensity is between 0.3 and 3.4 amperes.Additionally, the invention is concerned with an ignition system forpiston engines consisting of a battery or another current source, adistributor and a number of spark plugs corresponding to the number ofcylinders of the engine. The combination of the spark plugs asmake-and-break spark plugs and that each spark plug has associatedtherewith a thyristor which periodically delivers a low voltage pulse tothe associated spark plug is considered to be novel.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS Describing now the drawings, FIG. 1 illustrates a preferredembodiment of inventive spark plug which will be seen to comprise ahousing or shell 1 formed of an electrically conductive non-magneticmetal and essentially possesses the external configuration of a steppedcircular cylinder. Housing 1 encloses a continuous hollow bore orcompartment la which widens at its upper region into a compartment orchamber 2.

The lower housing portion possesses an external threading 3 by means ofwhich housing 1, and specifically by further using a sealing ring 4 asemployed with conventional spark plugs, can be threadably connected intoa threaded bore provided at the cylinder head of the engine. Thelowermost portion of the housing bore la is considerably narrowed sothat there only remains a central throughpassage 5.

Continuing, it will be seen that a cylindrical insulating sleeve 6 ismounted within the housing 1, above which sleeve there is located a micainsulating disc 7. A soft iron body member 8 constructed in the form ofa stepped cylinder body bears against the insulating disc 7, the lowerportion of soft iron body 8 being surrounded by the upper portion of thehousing 1. The uppermost edge 9 of the housing wall is inwardly flexedso that the soft iron body member 8 is fixedly clamped within thehousing 1. An insulating ring 10 as well as a further insulating disc 11prevents all electrical contact between the soft iron body member 8 andthe housing 1.

A continuous bore 8a piercingly extends through the soft iron bodymember 8, this continuous bore 8a having an upper somewhat widened boresection 12. in which there is accommodated a helical spring 13. Thelower end of the helical spring 13 bears upon the substantiallyring-shaped stepped surface of the bore. In the assembled condition ofthe device as shown in FIG. 2, the upper end of the helical spring 13continuously presses against the lower ring-shaped surface of a circularsubstantially cylindrical armature 14 fabricated from soft iron. Thisarmature 14 is rigidly connected with a substantially circular or roundrod member 15 fabricated from a non-magnetizable yet electricallyconductive material. Rod member 15 extends through the respectivecoaxial central bores 8a and 6a of the soft iron body member 8 and theinsulating sleeve 6 and carries at its lower end an electrode 16. In theillustration of FIG. 1, the circular rod 15 with the armature 14 hasbeen omitted to preserve clarity in illustration. The electrode 16 isconstructed in the form of a hollow cylinder, from the inner wall ofwhich there extends up to its central region a web 17, and at thatlocation is rigidly connected with the lower end of the rod member 15.The substantially ring-shaped end surfaces of the shell or housing 1 andthe electrode 16 which confront one another are preferably ground flatand size-matched to one another.

The armature 14, the circular rod member 15 and the electrode 16 possessa pressure equalization bore means 31 which communicates the interior ofthe engine cylinder with a pressure equalization chamber or compartment21 disposed above the armature 14. The pressure increase brought aboutby the piston ascending within its cylinder acts, on the one hand, frombelow upon the lower ring-shaped surface of the electrode 16 and, on theother hand, acts from above upon the end face of the armature 14. If theelectrode 16 is raised from its counter electrode or seat 1b then thepressure equalizes from the inside via the ring-shaped opening 30 in theelectrode 16 and, of course, also from the outside, so that only thepressure differential of the cross-section of the throughbored circularrod member 15 acts upon the electrode 16, whereby the pressure effect ismaintained at the armature 14.

The uppermost portion or section of the soft iron body member 8 extendsinto the internal compartment of a magnetic coil 18 which is mounted ina coil support 19 and closed towards the outside by a protective cap orsleeve 20. The coil support 19 consists of an electrically conductive,yet non-magnetizable metal and is threadablyconnected, while interposinga ring-shaped seal 27 with the upwardly extending section 12 of the softiron body member 8 equipped with external threading 12a. The pressureequalization compartment remaining above the armature 14 possesses aheight h which is greater than the height of the electrode 16, so thateven if there occurs an extensive erosion or burning-off of theelectrode 16 it still possesses a sufficient size in the sense of thedescribed pressure play.

The coil winding 18 is connected at location 22 to the metallic coilsupport 19. The other end of this coil winding 18 is connected via aconnection pin 23 and a plate 24.riveted therewith to a connection pin25. The plate 24 is electrically insulated by an insulating disc 26 withrespect to the coil support 19.

The length of the hollow circular rod is dimensioned such that in theillustrated rest portion of the electrode a certain amount of play aexists between the lower edge of the armature 14 and the upper edge ofthe therebelow situated soft iron body member 8, this play acorresponding to the maximum size of the spark gap.

The mode of operation of the complete ignition system is as follows:

By referring to FIG. 3 it will be understood that the voltage of aconventional automobile battery 32 is stepped up to a value exceedingvolts by means of a suitable transformer 33. A number of thyristors 34corresponding to the number of cylinders of the engine are electricallyconnected on the one hand with the transformer 33 and, on the otherhand, with the connection pin 25 of each respective associated sparkplug. The thyristors 34 are controlled by a distributor 35 which, inknown manner, for four-cycle engines delivers, for each second enginerevolution, a square-wave control pulse to each thyristor 34. Normallythe square-wave control pulse will possess a pulse width of onethreethousandths of a second. Triggering of the pulses occurs in knownmanner, for instance by a revolving magnet. Mechanically moved contactsare generally not desired, yet could be possibly utilized in theexisting installation. Upon the arrival of a squarewave pulse thecorresponding thyristor 34 opens or fires so that a current of about 0.5to 2 amperes flows via the connection pin 25 through the magnetic coil18 and via the soft iron body member 8 as well as the circular rodmember 15 to the electrode 16 and via the short-circuit path between theelectrode 16 to the spark plug body to ground. The magnetic field whichbuilds-up produces a downwardly directed force in the showing of thedrawing at the armature 14 by means of the soft iron body member 8. Whenthis force overcomes the pressure of the helical spring 13 and thealready described difference of the compression force, then, theelectrode 16 is catapulted in the direction of the interior of theengine cylinder. As a result, there generally occurs a series ofignition sparks which distribute themselves about the ring-shapedsurface of the electrode 16 and which are supplied from the stillflowing current and from the collapsing magnetic field of the magneticcoil 18. Upon collapse of the magnetic field the helical spring 13 againforces the armature 14 back into its starting position, where theelectrode 16 again bears against the counter electrode formed by thespark plug housing.

A particular advantage of the spark plug of the invention resides in theability to be able to choose the relationship of an automatic regulationof the firing time as a function of the pressure play. The point of timeof tearing away of the short-circuit path between the electrode 16 andthe counter electrode, on the one hand, depends upon the occurrence withtime of the control pulses at the associated thyristor 34, however,also, on the other hand upon the course of the pressure build-up in thecylinder of the motor, which can be used for improved combustion.

Experiments have shown that when using spark plugs designed according tothe teachings of the present invention there exists a considerablereduction in the content of the carbon monoxide in the exhaust gases, sothat when inspecting the automobile no significant thought need be givenabout permitting an engine equipped with spark plugs designed accordingto the invention to run in a closed area.

The bum-ofi' or erosion at the electrode is still nonetheless acceptablenotwithstanding the pronounced spark formation. During a trial run itwas found that upon travelling 15,000 kilometers there existed a bum-offor erosion of 2.5 millimeters at the electrode 16 and 2.5 millimeters atthe counter electrode.

During replacement the upper portion of the housing is threaded awayfrom the external threading at the section 12 of the soft iron bodymember 8 and the lower portion of the spark plug is exchanged.

Those skilled in the art will be readily capable of undertaking suitablemodifications and accommodating the spark plugs to particularconditions.

By virtue of the fact that the contact surfaces 1b and 16a orshort-circuit path between the movable electrode 16 and the counterelectrode formed by surface 1b are constructed to possess asubstantially ringshaped configuration there results a particularlyuniform erosion or bum-off. If any given location is somewhat morepronouncedly burned-off, then at that location ignition sparks no longerform for a certain time whereas protruding parts constitute preferredlocations for the spark formation. Moreover, the saving in fuel withengines equipped with spark plugs designed according to the presentinvention is considerable and in certain instances exceeds 10 percent.

While there is shown and described present preferred embodiments of theinvention, it is to be distinctly understood that the invention is notlimited thereto but may be otherwise variously embodied and practicedwithin the scope of the following claims.

What is claimed is:

1. A make-and-break spark plug for piston engines comprising a sparkplug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, the height of said pressure equalizationcompartment being greater than the height of said movable electrode, andmeans providing at least one bore for communicating said pressureequalization compartment with the interior of an engine cylinderassociated with the spark plug, said means providing said at least onebore extending through said armature, said connecting means for saidarmature with said movable electrode, and through both of saidelectrodes.

2. A make-and-break sparkplug for piston engines comprising a spark plughousing equipped with means providing a counter electrode, a movableelectrode positioned to cooperate with said counter electrode, saidmovable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said movable electrode and said counterelectrode possessing confronting contact surfaces forming therebetweensaid short-circuit path and possessing a substantially ring-shapedconfiguration.

3. The spark plug as defined in claim 1, further including an ignitioncircuit for such sparkplug embodying thyristor means for producingignition pulses.

4. The sparkplug as defined in claim 1, wherein said armature and saidmovable electrode are arranged for substantially coaxial movement.

5. The spark plug as defined in claim 4, wherein said movable electrodeis displaced away from said counter electrode in a direction awayfromsaid spark plughousing to break-up said short-circuit path.

6. A make-and-break spark plug for piston engines comprising a sparkplug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashorbcircuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said movable electrode being displacedaway from said counter electrode in a direction away from said sparkplug housing to breakup said short-circuit path, said movable electrodeextending past the periphery of said means for connecting said armaturewith said movable electrode, said movable electrode incorporating aprotruding marginal portion which in the rest position of said movableelectrode bears against a neighboring end contact surface of said sparkplug housing providing said counter electrode, said bore being providedat said armature, at said connecting means and at said movable electrodeto permit the pressure prevailing in the engine cylinder to betransmitted to said pressure equalization'compartment and to thussupport the axial movement of said movable electrode, and .ignitioncircuit means for delivering thyristor-controlled current pulses as afunction of the rotational speed of the engine to-the magneticcoil, sothat between the movable electrode and said neighboring end contact faceof said spark plug housing there exists an ignition spark whichcontinuously wanders upon the surface of said movable electrode.

7. A make-and-break spark plug for piston engines comprising a sparkplug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said armature being provided with acentral bore, said connecting means comprising a through-bored rodmember seated in said central bore and connected with said movableelectrode.

8. The spark plug as defined in claim 7, wherein said movable electrodeconsists of a substantially ringshaped member having a radiallyextending web to which there is secured said rod member.

9. A make-and-break spark plug for piston engines comprising a sparkplug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughous ing at the region of said end face of said armature facing awayfrom said movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said electrode being formed of a highmolecular weight material having a melting point exceeding 3,000 C.

10. The spark plug as defined in claim 9, wherein said electrode isformed of a material selected from molybdenum and molybdenum alloys.

11. The spark plug as defined in claim 1, wherein said housing of saidspark plug is constructed as a bipartite member, one part of whichhouses said magnetic coil, whereas the other part of which is anexchangeable housing portion which can be threadably connected with theengine cylinder and can be replaced after pronounced bum-off of themovable electrode.

12. An ignition system for piston engines comprising a power source, adistributor in circuit with said power source, a number of spark plugscorresponding to the number of cylinders of the engine, transformermeans for stepping up the voltage of the power source so as to exceed 30volts, said spark plugs comprising make-andbreak spark plugs, each suchspark plug including a magnetic coil and a movable electrode and acooperating counter electrode forming therebetween a short circuit path,a thyristor in circuit with the magnetic coil of each spark plug andsaid transformer means for periodically connecting in pulse-like fashionas a function of the rotational speed of the engine said transformermeans with the series circuit consisting of the magnetic coil of theassociated spark plug and the breakable short-circuit path between themovable electrode and the counter electrode.

1. A make-and-break spark plug for piston engines comprising a sparkplug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, the height of said pressure equalizationcompartment being greater than the height of said movable electrode, andmeans providing at least one bore for communicating said pressureequalization compartment with the interior of an engine cylinderassociated with the spark plug, said means providing said at least onebore extending through said armature, said connecting means for saidarmature with said movable electrode, and through both of saidelectrodes.
 2. A make-and-break spark plug for piston engines comprisinga spark plug housing equipped with means providing a counter electrode,a movable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forconnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said movable electrode and said counterelectrode possessing confronting contact surfaces forming therebetweensaid short-circuit path and possessing a substantially ring-shapedconfiguration.
 3. The spark plug as defined in claim 1, furtherincluding an ignition circuit for such spark plug embodying thyristormeans for producing ignition pulses.
 4. The spark plug as defined inclaim 1, wherein said armature and said movable electrode are arrangedfor substantially coaxial movement.
 5. The spark plug as defined inclaim 4, wherein said movable electrode is displaced away from saidcounter electrode in a direction away from said spark plug housing tobreak-up said short-circuit path.
 6. A make-and-breaK spark plug forpiston engines comprising a spark plug housing equipped with meansproviding a counter electrode, a movable electrode positioned tocooperate with said counter electrode, said movable electrode andcounter electrode forming therebetween a short-circuit path, an armaturemovably arranged within said spark plug housing, a magnetic coil foracting upon said armature, means for connecting said armature with saidmovable electrode, said armature including an end face directed awayfrom said movable electrode, means providing a pressure equalizationcompartment within said spark plug housing at the region of said endface of said armature facing away from said movable electrode, and meansproviding at least one bore for communicating said pressure equalizationcompartment with the interior of an engine cylinder associated with thespark plug, said means providing said at least one bore extendingthrough said armature, said connecting means for said armature with saidmovable electrode, and through both of said electrodes, said movableelectrode being displaced away from said counter electrode in adirection away from said spark plug housing to break-up saidshort-circuit path, said movable electrode extending past the peripheryof said means for connecting said armature with said movable electrode,said movable electrode incorporating a protruding marginal portion whichin the rest position of said movable electrode bears against aneighboring end contact surface of said spark plug housing providingsaid counter electrode, said bore being provided at said armature, atsaid connecting means and at said movable electrode to permit thepressure prevailing in the engine cylinder to be transmitted to saidpressure equalization compartment and to thus support the axial movementof said movable electrode, and ignition circuit means for deliveringthyristor-controlled current pulses as a function of the rotationalspeed of the engine to the magnetic coil, so that between the movableelectrode and said neighboring end contact face of said spark plughousing there exists an ignition spark which continuously wanders uponthe surface of said movable electrode.
 7. A make-and-break spark plugfor piston engines comprising a spark plug housing equipped with meansproviding a counter electrode, a movable electrode positioned tocooperate with said counter electrode, said movable electrode andcounter electrode forming therebetween a short-circuit path, an armaturemovably arranged within said spark plug housing, a magnetic coil foracting upon said armature, means for connecting said armature with saidmovable electrode, said armature including an end face directed awayfrom said movable electrode, means providing a pressure equalizationcompartment within said spark plug housing at the region of said endface of said armature facing away from said movable electrode, and meansproviding at least one bore for communicating said pressure equalizationcompartment with the interior of an engine cylinder associated with thespark plug, said means providing said at least one bore extendingthrough said armature, said connecting means for said armature with saidmovable electrode, and through both of said electrodes, said armaturebeing provided with a central bore, said connecting means comprising athrough-bored rod member seated in said central bore and connected withsaid movable electrode.
 8. The spark plug as defined in claim 7, whereinsaid movable electrode consists of a substantially ring-shaped memberhaving a radially extending web to which there is secured said rodmember.
 9. A make-and-break spark plug for piston engines comprising aspark plug housing equipped with means providing a counter electrode, amovable electrode positioned to cooperate with said counter electrode,said movable electrode and counter electrode forming therebetween ashort-circuit path, an armature movably arranged within said spark plughousing, a magnetic coil for acting upon said armature, means forcoNnecting said armature with said movable electrode, said armatureincluding an end face directed away from said movable electrode, meansproviding a pressure equalization compartment within said spark plughousing at the region of said end face of said armature facing away fromsaid movable electrode, and means providing at least one bore forcommunicating said pressure equalization compartment with the interiorof an engine cylinder associated with the spark plug, said meansproviding said at least one bore extending through said armature, saidconnecting means for said armature with said movable electrode, andthrough both of said electrodes, said electrode being formed of a highmolecular weight material having a melting point exceeding 3,000* C. 10.The spark plug as defined in claim 9, wherein said electrode is formedof a material selected from molybdenum and molybdenum alloys.
 11. Thespark plug as defined in claim 1, wherein said housing of said sparkplug is constructed as a bipartite member, one part of which houses saidmagnetic coil, whereas the other part of which is an exchangeablehousing portion which can be threadably connected with the enginecylinder and can be replaced after pronounced burn-off of the movableelectrode.
 12. An ignition system for piston engines comprising a powersource, a distributor in circuit with said power source, a number ofspark plugs corresponding to the number of cylinders of the engine,transformer means for stepping up the voltage of the power source so asto exceed 30 volts, said spark plugs comprising make-and-break sparkplugs, each such spark plug including a magnetic coil and a movableelectrode and a cooperating counter electrode forming therebetween ashort-circuit path, a thyristor in circuit with the magnetic coil ofeach spark plug and said transformer means for periodically connectingin pulse-like fashion as a function of the rotational speed of theengine said transformer means with the series circuit consisting of themagnetic coil of the associated spark plug and the breakableshort-circuit path between the movable electrode and the counterelectrode.